OK, here we are. I performed bit-comparisons of different -q values under -V0.
As documented, -q0 to -q4 are identical. However, -q5 to -q9 are not identical as the documentation and comments claim. Specifically, -q5 and -q6 are identical, but -q7 to -q9 form a separate group (internally identical).
Thus, in reality, VBR seems to provide (at least) three choices of -q, not the two claimed. Again, I would appreciate clarification on this from a developer. If it is intended, the documentation should be updated.
All tracks decoded fine, no differences found.
Comparing:
"C:\Users\?\Downloads\piano2q0.mp3"
"C:\Users\?\Downloads\piano2q1.mp3"
No differences in decoded data found.
Differences found in 1 out of 1 track pairs.
Comparing:
"C:\Users\?\Downloads\piano2q4.mp3"
"C:\Users\?\Downloads\piano2q5.mp3"
Differences found: 7518 sample(s), starting at 0.3370000 second(s), peak: 0.0023986 at 2.0611458 second(s), 1ch
All tracks decoded fine, no differences found.
Comparing:
"C:\Users\?\Downloads\piano2q5.mp3"
"C:\Users\?\Downloads\piano2q6.mp3"
No differences in decoded data found.
Differences found in 1 out of 1 track pairs.
Comparing:
"C:\Users\?\Downloads\piano2q6.mp3"
"C:\Users\?\Downloads\piano2q7.mp3"
Differences found: 605424 sample(s), starting at 0.0000000 second(s), peak: 0.0077936 at 2.4563750 second(s), 1ch
All tracks decoded fine, no differences found.
Comparing:
"C:\Users\?\Downloads\piano2q7.mp3"
"C:\Users\?\Downloads\piano2q8.mp3"
No differences in decoded data found.
All tracks decoded fine, no differences found.
Comparing:
"C:\Users\?\Downloads\piano2q8.mp3"
"C:\Users\?\Downloads\piano2q9.mp3"
No differences in decoded data found.
I then captured the verbose output of the console to get a better idea of what is causing the differences. There are even more differences in applied settings than the bit-comparisons reveal! Specifically, -q5 and -q6, whilst apparently producing identical output (at least on this sample), invoke different sets of parameters for noise-shaping. Again, data below ascend numerically, and data not shown for higher values of -q are identical to those used by the previous setting.
-q0 to -q4:
misc:
scaling: 1
ch0 (left) scaling: 1
ch1 (right) scaling: 1
huffman search: best (outside loop)
experimental Y=0
psychoacoustic:
using short blocks: channel coupled
subblock gain: 1
adjust masking: -6.8 dB
adjust masking short: -6.8 dB
quantization comparison: 9
^ comparison short blocks: 9
noise shaping: 1
^ amplification: 2
^ stopping: 1
ATH: using
^ type: 5
^ shape: 1 (only for type 4)
^ level adjustement: -7.1 dB
^ adjust type: 3
^ adjust sensitivity power: 1.000000
experimental psy tunings by Naoki Shibata
adjust masking bass=-0.5 dB, alto=-0.25 dB, treble=-0.025 dB, sfb21=8
.25 dB
using temporal masking effect: no
interchannel masking ratio: 0
-q5
misc:
huffman search: normal
psychoacoustic:
noise shaping: 1
^ amplification: 2
^ stopping: 1
-q6
psychoacoustic:
noise shaping: 1
^ amplification: 0
^ stopping: 0
-q7
psychoacoustic:
subblock gain: -1
noise shaping: 0
^ amplification: 0
^ stopping: 0
Devs? Anyone?